Study On The Purification Of Ice From Suspension Freezing Process Based On The Solid-liquid Separation Principle

Suspension freezing has been applied to separate ice crystals from pollutants basing on principle of equilibrium between the solid and liquid phases.However,the poor separation efficiency for ice and concentrate often leads to contaminant in ice crystals and reduce ice purity.In this work,the ice was produced from NaCl wastewater by suspension freezing process.The solid-liquid separation methods including gravity,vacuum filtration and centrifugation were used for purification of ice.Effects of various factors on the Cl-removal efficiency,conductivity removal efficiency and dewatering rate were investigated,meanwhile mechanisms of ice purification were discussed.Influences of time,ice thickness and temperature on the ice purification by gravity were investigated.Results showed that the Cl-and conductivity removal efficiencies and dewatering rate presented an increasing trend with the increase of filtration time and ambient temperature,while the ice purification and the dewatering rate gradually decreased as the ice thickness increased.There was a positive linear relationship between the removal efficiencies and dewatering rate with the R2 above 0.9.When the ice was filtered for 50 min at the thickness of 7.5 cm and the temperature of 25℃,the removal efficiencies of Cl-and conductivity reached 80.4%and 78.5%with the dewatering rate of 40.3%.The total energy consumption for treating 1 L ice by gravity method was 4.82 × 10-4 kW·h.Mechanism revealed the brine on the surface of the ice crystals was discharged by flushing,meanwhile the brine in ice gap was discharged along the channel by gravity.Also,Ostwald ripening phenomenon create the increase of ice size.Also,vacuum filtration was used to purify ice crystals.Results indicated that filtration time,ice thickness and ambient temperature generated affects on the purification of ice.When the ice was filtered for 30 min at the thickness of 7.5 cm and the temperature of 25℃,the removal efficiencies of Cl-and conductivity reached 93.7%and 89.2%,and the dewatering rate was 61.5%.Mechanism suggested that brine on the surface and inside the ice crystals was removed in time with the action of Ostwald ripening phenomenon and squeezing pressure by vacuum filtration,which was more effective and less time consuming than the gravity method.Finally,effect of centrifugation on the purification of ice was investigated.Influences of centrifugal time,centrifugal speed,ice thickness and centrifugal force on the ice purification were researched.Results suggested that the removal efficiencies of Cl-and conductivity and dewatering rate increased with increasing centrifugal time and speeds.When the ice was centrifuged for 5 min at the speed of 3000 r/min,the Cl-and conductivity removal efficiencies reached 83.3%and 81.7%,and the dewatering rate was 67.1%.The mechanistic studies showed that water in ice crystals was removed in the form of free water,capillary water and internal water in order during centrifugation.Centrifugation speed of 500 r/min performed the high efficiency for Cl-removal in unit centrifugal force and reached a maximum of 2.082%.Compared with gravity and vacuum filtration methods,centrifugation had the highest Cl-and conductivity removal efficiencies of 39.9%and 39.2%per unit time.